<template>
        <ul class="list">
            <li 
                class="item"
                v-for="key of letters"
                :key="key"
                :ref="key"
                @touchstart="handleTouchStart"
                @touchmove="handleTouchMove"
                @touchend="handleTouchEnd"
                @click="handleLetterClick"
              >
                {{key}}
              </li>
        </ul>
</template>
<script>
export default {
    name: "CityAlphabet",
    props: {
         cities: Object
    },
    computed: {
        letters () {
            const letters = []
            for (let i in this.cities) {
                letters.push(i)
            }
           
            return letters
        }
    },
    data () {
        return {
            touchStatus: false,
            startY: 0,
            timer: null
        }
    },
    updated () {
         this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
        handleLetterClick (e) {
            //触发事件
            this.$emit('change', e.target.innerText)
        },
        handleTouchStart () {
            this.touchStatus = true
        },
        handleTouchMove (e) {
      if (this.touchStatus) {
        if (this.index) {
          clearTimeout(this.timer)
        }
        this.timer = setTimeout(() => {
          const touchY = e.touches[0].clientY -79
          const index = Math.floor((touchY - this.startY) / 20)
    
          if (index >= 0 && index < this.letters.length) {
            this.$emit('change', this.letters[index])
          }
        }, 16)
      }
    },

        handleTouchEnd () {
            this.touchStatus = false
        }
    }
}
</script>
<style lang="stylus" scoped>
    @import '~style/varibies.styl'
    .list
        //让字母垂直居中
        display flex
        flex-direction column
        justify-content center
        position absolute
        top 0
        right 0
        bottom 0
        width .4rem
      .item
         line-height .4rem
         text-align center
         color $bgColor
</style>